'Kickbacks'

"His family members, colleagues, friends and the public at large have shown annoyance and grievances," Justice Chaudhry said.

"And according to them, they are not expecting a free, fair and honest investigation because of the involvement of highly influential political and executive authorities."

Two Supreme Court judges will hold their first hearing into Mr Faisal's death on Thursday. Their investigation will operate in parallel to the judicial inquiry ordered by the interior ministry on Sunday.

Some police investigators say Mr Faisal may have been on anti-depressants at the time he died, which might support claims he took his life. But his family say his body bore marks of torture, suggesting he was murdered.

Mr Faisal had been helping lead the investigation into alleged bribes paid by power firms when Mr Ashraf was minister for water and power in 2010. The prime minister denies wrongdoing.

Last week, the Supreme Court ordered his arrest along with 15 officials also accused in the so-called Rental Power Projects case.

But the prime minister appears unlikely to be detained soon - the head of the NAB has said there is not enough evidence to justify such a move.

The bureau has suspended its investigation into the case, pending the outcome of the inquiries into the death of Mr Faisal.